It is known to provide optically variable devices in which arrays of lenticular (part-cylindrical lenses) focus on an object plane containing multiple sets of interleaved image elements. Each set of image elements (strips) belongs to a distinct image, so that as the person viewing the device changes the angle of view, a different image becomes visible. Devices including non-cylindrical lenses, for example those including two-dimensional arrays of spherical microlenses, are also known.
In security applications, and in particular when dealing with flexible security documents such as banknotes, it is desirable to minimise the thickness of a lens array applied to the security document. Known lenticular devices as described above, being relatively thick (hundreds of microns), are unsuitable for application to flexible security documents.
To avoid adding significantly to the thickness of a banknote, microlenses which have a relatively small focal length, and which must therefore be of relatively small transverse dimension (perhaps of the order of 50-65 microns or less) are desirable. Lenses of this size place significant constraints on the processes which can be used to apply the image elements to the object plane. For example, gravure printing (sometimes known as rotogravure printing) can presently only consistently produce printed line widths of 35 microns or more. With this line width, lenses of transverse dimension 65 microns are only sufficiently wide to enable implementation of very simple optically variable effects such as monochromatic flipping images, in which the contrast of the image switches from positive to negative as the viewing angle changes. More complex multi-frame effects are not possible via gravure printing due to the limitation on minimum gravure-printed feature size.
It has been found that simple effects such as the flipping images described above can easily be counterfeited solely using printing techniques. Such effects therefore have limited security value. Their security value could potentially be increased by printing lines that are significantly smaller than 35-45 microns. However, the difficulty has been that it is not possible to reliably print lines significantly smaller than 35-45 microns using traditional gravure printing.
In view of the difficulties described above, it is desirable to provide a security device which is more resistant to counterfeiting, yet which can be produced by a wider variety of security printing techniques, including gravure printing.
Definitions
Focal Point Size H
As used herein, the term focal point size refers to the dimensions, usually an effective diameter or width, of the geometrical distribution of points at which rays refracted through a lens intersect with an object plane at a particular viewing angle. The focal point size may be inferred from theoretical calculations, ray tracing simulations, or from actual measurements.
Focal Length f
In the present specification, focal length, when used in reference to a microlens in a lens array, means the distance from the vertex of the microlens to the position of the focus given by locating the maximum of the power density distribution when collimated radiation is incident from the lens side of the array (see T. Miyashita, “Standardization for microlenses and microlens arrays” (2007) Japanese Journal of Applied Physics 46, p 5391).
Gauge Thickness t
The gauge thickness is the distance from the apex of a lenslet on one side of the transparent or translucent material to the surface on the opposite side of the translucent material on which the image elements are provided which substantially coincides with the object plane.
Lens Frequency and Pitch
The lens frequency of a lens array is the number of lenslets in a given distance across the surface of the lens array. The pitch is the distance from the apex of one lenslet to the apex of the adjacent lenslet. In a uniform lens array, the pitch has an inverse relationship to the lens frequency.
Lens Width W
The width of a lenslet in a microlens array is the distance from one edge of the lenslet to the opposite edge of the lenslet. In a lens array with hemispherical or semi-cylindrical lenslets, the width will be equal to the diameter of the lenslets.
Radius of Curvature R
The radius of curvature of a lenslet is the distance from a point on the surface of the lens to a point at which the normal to the lens surface intersects a line extending perpendicularly through the apex of the lenslet (the lens axis).
Sag Height s
The sag height or surface sag s of a lenslet is the distance from the apex to a point on the axis intersected by the shortest line from the edge of a lenslet extending perpendicularly through the axis.
Refractive Index n
The refractive index of a medium n is the ratio of the speed of light in vacuo to the speed of light in the medium. The refractive index n of a lens determines the amount by which light rays reaching the lens surface will be refracted, according to Snell's law:ni*Sin(α)=n*Sin(θ),where α is the angle between an incident ray and the normal at the point of incidence at the lens surface, θ is the angle between the refracted ray and the normal at the point of incidence, and n1 is the refractive index of air (as an approximation n1 may be taken to be 1).
Conic Constant P
The conic constant P is a quantity describing conic sections, and is used in geometric optics to specify spherical (P=1), elliptical (0<P<1, or P>1), parabolic (P=0), and hyperbolic (P<0) lens. Some references use the letter K to represent the conic constant. K is related to P via K=P−1.
Lobe Angle
The lobe angle of a lens is the entire viewing angle formed by the lens.
Abbe Number
The Abbe number of a transparent or translucent material is a measure of the dispersion (variation of refractive index with wavelength) of the material. An appropriate choice of Abbe number for a lens can help to minimize chromatic aberration.
Security Document
As used herein, the term security document includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licences, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.
Transparent Windows and Half Windows
As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.
A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.
A partly transparent or translucent area, hereinafter referred to as a “half-window”, may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the “half-window” is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.
Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.
Opacifying Layers
One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that
LT<L0, where L0 is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.